Question

Which parts of this excerpt from Christopher Marlowe’s The Tragical History of Doctor Faustus bring out the theme of forbidden knowledge?

Answer 1

"Whose deepness doth entice such forward wits / To practice more than heavenly power permits".
These lines bring out the theme of forbidden knowledge. 

Answer 2

I believe the answer is: "Whose deepness doth entice such forward wits / To practice more than heavenly power permits".

The keywords are 'wits' and 'permits'

Wits means a certain knowledge or teaching, and the fact that the wit is more than something that heavenly power permits indicates that you are not supposed to have that 'wits'